One of the therapeutic procedures applicable to the present invention is known as percutaneous transluminal coronary angioplasty (PTCA). This procedure can be used, for example, to reduce arterial build-up of cholesterol fats or atherosclerotic plaque. Typically a guidewire is steered through the vascular system to the site of therapy. A guiding catheter, for example, can then be advanced over the guidewire and a balloon catheter advanced within the guiding catheter over the guidewire. The balloon at the distal end of the catheter is inflated causing the site of the stenosis to widen. The original catheter can then be withdrawn and a catheter of a different size or another device such as an atherectomy device can be inserted.
The design of medical devices for insertion into body organs has always involved trading off various performance characteristics in the design of a satisfactory implement. PTCA requires a device which is stiff enough to be pushable and go through blockage, while being flexible enough to go around bends. Dilatation balloon catheters commonly have a guidewire lumen pass through the balloon with the balloon and guidewire lumen being bonded at the distal end. This makes the tip of the dilatation catheter a relatively rigid structure which is desired for pushing the dilatation balloon catheter through a stenosis or blocked artery. If the tip of the catheter is too stiff, however, it may not be able to navigate sharp turns to get to the stenosis. This limits the effectiveness of the dilatation balloon catheter to arteries within easy reach of the heart.
Conventional angioplasty balloons fall into high, medium and low pressure ranges. Low pressure balloons are those which fall into rated burst pressures below 6 atm. Medium pressure balloons are those which fall into rated burst pressures between 6 and 12 atm. High pressure balloons are those which fall into rated burst pressures above 12 atm. Burst pressure is determined by such factors as wall thickness and tensile strength.
High pressure balloons are desirable because they have the ability to exert more force and crack hard lesions. High pressure balloons are useful in stent deployment. A stent is a half-inch stainless-steel or tantalum mesh sleeve that props open blocked coronary arteries, keeping them from reclosing after balloon angioplasty. A balloon of appropriate size and pressure is first used to open the lesion. The process is repeated with a stent crimped on a high pressure balloon. The stent is deployed when the balloon is inflated. The stent remains as a permanent scaffold after the balloon is withdrawn. A high pressure balloon is necessary for stent deployment because the stent must be forced against the artery's interior wall so that it will fully expand thereby precluding the ends of the stent from hanging down into the channel encouraging the formation of thrombus.
High pressure balloon materials are stiffer than conventional materials. Whereas conventional balloons use materials such as polyethylene, high pressure balloons use materials such as polyethylene terephthalate (PET) or Nylon 12. When high pressure balloon materials are used, the balloon tip can become too stiff and will not negotiate tortuous paths as well as more compliant conventional balloons.
Copending commonly held U.S. Ser. No. 08/312,359 to Ma for "Catheter Flexible Tip" discloses a flexible tip formed from balloon material distally extending beyond the guidewire lumen, the inner diameter of the flexible tip generally equal to the inner diameter of the guidewire lumen.
U.S. Pat. No. 4,739,768 to Engelson for "Catheter for Guide-Wire Tracking" discloses a drug delivery catheter with a relatively stiff proximal segment and a relatively flexible distal segment that is at least 5 cm long. It can be advanced along a guidewire placed in a tortuous vascular path and provides a method for delivery of an injectable fluid at a tissue site. While the distal segment itself may be very flexible and works well for drug delivery, it is too long and flexible to push through a blockage in the artery or stenosis.
In Ma and Engelson the balloon material extends beyond the inner lumen thereby forming the balloon tip. Using high pressure balloon materials for such a tip design yields a tip which is too stiff.